Effect of Inclusion Complexation with Cyclodextrin on the Cloud Point

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Effect of Inclusion Complexation with Cyclodextrin on the Cloud Point of Poly(2-(dimethylamino)ethyl methacrylate) Solution Yu Zhao, Kai Guo, Chunyu Wang,* and Liyan Wang* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China Received December 18, 2009. Revised Manuscript Received February 25, 2010 We investigated the influence of addition of cyclodextrin (CD) on the cloud point of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) aqueous solution. Both β-CD and randomly methylated β-cyclodextrin (RM-β-CD) raised the cloud point of PDMAEMA solution; by contrast, the addition of R-CD or γ-CD did not affect the cloud point of PDMAEMA solution. The inclusion complexation between side group of PDMAEMA and RM-β-CD was confirmed by NMR experiments. The cloud point of PDMAEMA solution was raised gradually with the increase of concentration of RM-β-CD. On the basis of inclusion complexation equilibrium, we deduced an equation, which clearly described the relationship between cloud point and concentration of RM-β-CD. This analysis will provide an important theoretic guide for other related research. In comparison with copolymerization with another monomer, such a supramolecular approach avoids a series of synthesis and facilitates fine-tuning of cloud point.

Introduction Stimuli-responsive polymers are polymers that have one or more properties that can be significantly changed in a controlled fashion by an external stimulus, such as metal ions, temperature, humidity, pH, and light irradiation.1,2 There has been extensive research on these polymers because of their potential applications in some fields, such as drug delivery, sensing materials, and fluidic switching devices.3-6 Thermosensitive polymers fall into the category of stimuliresponsive polymers.1,7 If a polymer solution has one phase below a specific temperature and is phase-separated above this temperature, the cloud point (Tcp) of the polymer solution is the temperature where the solution starts to phase-separate and become cloudy. Such a polymer generally has a lower critical solution temperature (LCST), the lowest temperature of the phase separation curve on concentration-temperature diagram.1 The cloud point of a thermosensitive polymer can be tuned by random copolymerization with another monomer.1,8 For example, the introduction of the relatively hydrophilic acrylamide raises the cloud point of poly(N-isopropylacrylamide) (PNIPAAm); by contrast, the introduction of the hydrophobic N-butylacrylamide lowers the cloud point.9 In addition, the cloud *Corresponding authors. Telephone: þ86-431-85168479, Fax: þ86-43185193421, E-mail: [email protected]; [email protected].

(1) Gil, E. S.; Hudson, S. A. Prog. Polym. Sci. 2004, 29, 1173–1222. (2) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1–62. (3) Schmaljohann, D. Adv. Drug Delivery Rev. 2006, 58, 1655–1670. (4) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357–360. (5) Ma, Y.; Sun, J. Chem. Mater. 2009, 21, 898–902. (6) Lokuge, I.; Wang, X.; Bohn, P. W. Langmuir 2007, 23, 305–311. (7) Wu, C.; Zhou, S. Macromolecules 1995, 28, 8381–8387. (8) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249. (9) Hoffman, A. S.; Stayton, P. S.; Bulmus, V.; Chen, G.; Chen, J.; Cheung, C.; Chilkoti, A.; Ding, Z.; Dong, L.; Fong, R.; Lackey, C. A.; Long, C. J.; Miura, M.; Morris, J. E.; Murthy, N.; Nabeshima, Y.; Park, T. G.; Press, O. W.; Shimoboji, T.; Shoemaker, S.; Yang, H. J.; Monji, N.; Nowinski, R. C.; Cole, C. A.; Priest, J. H.; Harris, J. M.; Nakamae, K.; Nishino, T.; Miyata, T. J. Biomed. Mater. Res., Part A 2000, 52, 577–586. (10) Zhang, Y. J.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505–14510. (11) Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948–952.

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point of a thermosensitive polymer solution is often affected by additives, such as salts,10 solvent,11,12 surfactants.13,14 The most notable feature of a cyclodextrin (CD) is the ability to capture various hydrophobic or amphiphilic species inside its hydrophobic cavity and form a host-guest inclusion complex.15 A CD is a good host not only for small molecules, but also for polymers. Harada and co-workers carried out a lot of studies on the complexation of polymer and CDs.16-18 Furthermore, CDs have been successfully used in adjusting self-assembling behaviors of amphiphiles.19-21 Although there are some papers about complexation between CDs and a thermosensitive polymer,22,23 only a few focus on the complexation between CDs and the side groups of a thermosensitive homopolymer. Recently, Schmitz and Ritter24 synthesized poly(N-(3-(dimethylamino)propyl)-methacrylamide). The polymer solution shows a thermosensitive property only at pH 14. After randomly methylated β-cyclodextrin (RM-β-CD) was added into the polymer solution, the cloud point of the solution was raised from 35 to 75 °C at most. In contrast, after Han and coworkers added β-CD into a solution of PNIPAAm, the cloud point of solution did not show a significant change.25 (12) Winnik, F. M.; Ottaviani, M. F.; Bossmann, S. H.; Garcia-Garibay, M.; Turro, N. J. Macromolecules 1992, 25, 6007–6017. (13) Schild, H. G.; Tirrell, D. A. Langmuir 1991, 7, 665–671. (14) Meewes, M.; Ricka, J.; De Silva, M.; Nyffenegger, R.; Binkert, T. Macromolecules 1991, 24, 5811–5816. (15) Saenger, W. Angew. Chem., Int. Ed. 1980, 19, 344–362. (16) Harada, A. Coord. Chem. Rev. 1996, 148, 115–133. (17) Harada, A.; Takashima, Y.; Yamaguchi, H. Chem. Soc. Rev. 2009, 38, 875– 882. (18) Wenz, G.; Han, B.-H.; Mueller, A. Chem. Rev. 2006, 106, 782–817. (19) Wang, Y. P.; Ma, N.; Wang, Z. Q.; Zhang, X. Angew. Chem., Int. Ed. 2007, 46, 2823–2826. (20) Park, C.; Lee, I. H.; Lee, S. H.; Song, Y.; Rhue, M.; Kim, C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1199–1203. (21) Jing, B.; Chen, X.; Wang, X.; Yang, C.; Xie, Y.; Qiu, H. Chem.;Eur. J. 2007, 13, 9137–9142. (22) Ritter, H.; Sadowski, O.; Tepper, E. Angew. Chem., Int. Ed. 2003, 42, 3171– 3173. (23) Alonso, M.; Arranz, D.; Reboto, V.; Rodrı´ guez-Cabello, J. C. Macromol. Chem. Phys. 2001, 202, 3027–3034. (24) Schmitz, S.; Ritter, H. Macromol. Rapid Commun. 2007, 28, 2080–2083. (25) Han, S. J.; Yoo, M. K.; Sung, Y. K.; Lee, Y. M.; Cho, C. S. Macromol. Rapid Commun. 1998, 19, 403–407.

Published on Web 03/08/2010

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Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was widely applied in gene delivery26 and self-assembly of block copolymer,27 and so forth. According to reports,28 PDMAEMA shows thermosensitive behavior in a broad pH range from 7 to 10. In this study, RM-β-CD was added into a PDMAEMA solution. We anticipate RM-β-CD will form inclusion complex with the side group of PDMAEMA, thus affecting the cloud point of PDMAEMA. The present work aims at elucidating the relationship between cloud point and concentration of RM-β-CD from a thermodynamic viewpoint.

Experimental Section Materials. RM-β-CD (average MW 1310, 1.6-2.0 methyl

per unit anhydroglucose), R-CD, azobisisobutyronitrile, and 2-(dimethylamino)ethyl methacrylate (DMAEMA) were purchased from Aldrich. γ-CD was obtained from J&K. β-CD was purchased from Sinopharm chemical reagent company. Tetrahydrofuran, petroleum ether, and other solvents were of analytical grade. Synthesis of PDMAEMA. DMAEMA (2.0 g) and azobisisobutyronitrile (0.04 g) were dissolved in 10 mL of freshly distilled tetrahydrofuran. The mixture was degassed by three freezepump-thaw cycles. Polymerization was carried out at 70 °C for 24 h. The polymer solution was precipitated in 1000 mL petroleum ether (boiling range 60-90 °C). The obtained polymer was purified by reprecipitation and dried in a vacuum at 45 °C (white powder). 1H NMR (500 MHz, D2O), δ 0.65 (3H, -C(CH3)CdO), 1.69 (-CH2-C(CH3)CdO), 2.06 (6H, -N(CH3)2), 2.47 (2H, -CH2-N(CH3)2), 3.89 (2H, -C(CH3)COOCH2-), 4.70 (HDO) ppm. The average molecular weights of PDMAEMA are Mn 2.24  104 and Mw 4.63  104 as measured by gel permeation chromatography.

Figure 1. Transmittance-temperature curves obtained in a buffer solution of pH 9.18. (a) 1.0 g/L (6.37  10-3 mol/L) PDMAEMA, Tcp = 33.5 °C; (b) 1.0 g/L PDMAEMA and 6.2 g/L (6.37  10-3 mol/L) R-CD, Tcp = 33.4 °C; (c) 1.0 g/L PDMAEMA and 7.23 g/L (6.37  10-3 mol/L) β-CD, Tcp = 35.9 °C; (d) 1.0 g/L PDMAEMA and 8.34 g/L (6.37  10-3 mol/L) RM-β-CD, Tcp = 36.0 °C; (e) 1.0 g/L PDMAEMA and 8.26 g/L (6.37  10-3 mol/L) γ-CD, Tcp = 33.6 °C.

Determination of Cloud Point of a Polymer Solution. Transmittance-temperature curves were achieved by measuring transmittance at 500 nm using UV-vis spectrometry (UV-3100, Shimadzu) with a thermostatic accessory. In addition, a temperature sensor (Pt 100) was used to measure the temperature of the solution in a glass cell. The temperature program (0.5 °C/min) was run by a PID temperature controller (708p, Xiamen Yudian) mounted on a thermostatic bath (THD-1006, Ningbo Tianheng). We defined the cloud point as the intercept of the tangent of a transmittance-temperature curve at the onset of turbidity (see Supporting Information Figure S1). NMR Measurements. 1H NMR and NOE NMR spectra were both recorded on a Bruker Avance-500 NMR spectrometry (500 MHz) in D2O using HDO (4.70 ppm) as an internal standard. Sample Preparation. Polymer solutions were prepared with sodium borate buffer solution, maintaining its pH around 9.18, unless otherwise stated. At this pH value, few of tertiary amino groups of PDMAEMA are protonated, for the pKb of PDMAEMA equals 6.9 (see Supporting Information). All solutions were stored at 5 °C for 12 h before the measurement of cloud point. A pH glass microelectrode (Shanghai Jingmi) was used as the pH sensor.

Results and Discussion Effect of Cyclodextrins on Cloud Points. First, we recorded a transmittance-temperature curve of 1.0 g/L PDMAEMA (the molar concentration of repeat unit is 6.37  10-3 mol/L) in the standard sodium borate buffer solution (pH = 9.18) during heating, and obtained a cloud point of 33.5 °C (Figure 1 curve (a)). (26) Cherng, J. Y.; van de Wetering, P.; Talsma, H. Pharmacol. Res. 1997, 14, 1838–1841. (27) Gohy, J. F.; Antoun, S.; Jerome, R. Macromolecules 2001, 34, 7435–7440. (28) Plamper, F. A.; Ruppel, M.; Schmalz, A.; Borisov, O.; Ballauff, M.; Mueller, A. H. E. Macromolecules 2007, 40, 8361–8366.

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Figure 2. 1H NMR spectrum of PDMAEMA (1.0 g/L) in D2O.

Then, we recorded a transmittance-temperature curve of a solution that contained 1.0 g/L PDMAEMA and 8.34 g/L RM-β-CD (6.37  10-3 mol/L), and obtained a cloud point of 36.0 °C (curve (d)). The addition of RM-β-CD raises the cloud point of PDMAEMA solution. We further studied the influence of different sorts of cyclodextrins on the cloud point of PDMAEMA solution, maintaining the molar ratio of polymer repeat unit to RM-β-CD at 1:1. The cloud points of curve (c) and curve (d) are almost the same, both higher than that of PDMAEMA. In contrast, the cloud points of curve (b) and curve (e) are approximately equal to that of PDMAEMA, so that the addition of R-CD or γ-CD has almost no effect on the cloud point of PDMAMEA solution. Thus, the influence of cyclodextrins on cloud point of PDMAEMA solution is selective on account of the difference in cavity sizes of these cyclodextrins, which implies formation of inclusion complexes. Since the solubility of RM-βCD is greater than that of β-CD, we use RM-β-CD in the following experiments. Inclusion Complexation between RM-β-CD and PDMAEMA. The inclusion complexation between RM-β-CD and PDMAEMA was studied by a gradient one-dimensional NMR selective NOE experiment. The proton peaks of PDMAEMA and RM-β-CD NMR spectra are identified, as shown in Figure 2 DOI: 10.1021/la9047819

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Figure 3. 1H NMR spectrum of RM-β-CD (8.34 g/L) in D2O. Figure 5. Job’s plot for inclusion complexation of PDMAEMA and RM-β-CD. The sum of the host concentration and guest concentration is 6.37  10-2 mol/L.

group rather than with polymer backbone (see Supporting Information). Stoichiometry of Inclusion Complexation. The inclusion stoichiometry of host-guest complex can be determined with Job’s method.30 In this case, RM-β-CD acts as host, while the repeating unit of PDMAEMA is the corresponding guest. Cloud point is taken as the property that is closely related with the concentration of complex. It is assumed that Tc ¼ ðT0 C þ Tc0 Cc Þ=C0

Figure 4. One-dimensional NMR selective NOE spectrum of PDMAEMA and RM-β-CD in D2O of pH 8.0 at 283 K.

and Figure 3. For the RM-β-CD spectrum, several proton peaks are roughly assigned,29 because the peaks overlap each other due to random methylation of RM-β-CD. In the one-dimensional NMR selective NOE experiment, the protons of the dimethylamino group (δ = 2.06 ppm) were irradiated, since the dimethylamino group was most likely to complex with RM-β-CD. As shown in Figure 4, strong NOE signals appears in the region of RM-β-CD (δ = 3.3-3.8 ppm), indicating the existence of interaction between dimethylamino group and RM-β-CD. Compared with the RM-β-CD spectrum in Figure 3, the one-dimensional selective NOE signals in Figure 4 change a lot. First, the relative intensity of inner wall protons of RM-β-CD (H-3, H-5) and outer wall protons (H-6, H-2, H-4) in Figure 4 is different from that in Figure 3. This implies that the protons on the inner wall of RM-β-CD are closer to the dimethylamino groups than those on the outer wall. Thus, we inferred that inclusion complexes of RM-β-CD and the side groups of PDMAEMA formed in the solution. Second, the relative intensity of H-7 peak to H-8 peak in Figure 4 is greater than that in Figure 3. Therefore, the dimethylamino group is closer to the secondary rim of cyclodextrin than to the primary rim. Additionally, 2D NOESY NMR experimental results, together with the NMR titration experimental results, indicate that DM-β-CD formed inclusion complex with polymer side (29) Storsberg, J.; Ritter, H. Macromol. Rapid Commun. 2000, 21, 236–241.

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ð1Þ

T0 is the cloud point of PDMAEMA in the absence of RM-βCD. Tc0 is the cloud point of PDMAEMA when all the repeat units of PDMAEMA are complexed with RM-β-CD (it is an imaginary state). Tc0 could reflect the intrinsic ability of RM-βCD to raise the cloud point of PDMAEMA solution. Tc is the actual cloud point of PDMAEMA solution containing different amounts of RM-β-CD. C0 is the initial concentration of the repeating units of PDMAEMA. Cc is the equilibrium concentration of the repeating units of PDMAEMA complexed with RM-β-CD. C is the concentration of the repeating units that are not complexed with RM-β-CD. That is to say C0 ¼ C þ Cc

ð2Þ

Introducing eq 2 into eq 1, we obtained Cc ¼ ðTc -T0 ÞC0 =ðTc0 -T0 Þ

ð3Þ

Because (Tc0 - T0) is a constant, the concentration of the complex Cc is proportional to (Tc - T0)C0. An appropriate Job’s plot for the present system would be (Tc - T0)C0 as a function of the molar concentration fraction of guest (Xg). Job’s method requires that the sums of the host concentration and guest concentration are equal in all measurements, so that C0 is proportional to Xg. Thus, we plot (Tc - T0)Xg as a function of Xg, as shown in Figure 5. When Xg equals 0.5, (Tc - T0)Xg has a maximum, i.e., the concentration of complex is maximal. This result indicates that the stoichiometry of inclusion complex of RM-β-CD and a repeating unit is 1.30 (30) Connors, K. A. Binding Constants: the Measurement of Molecular Complex Stability; Wiley: New York, 1987.

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Figure 6. Transmittance-temperature curves of PDMAEMA solutions (1.0 g/L, pH = 9.18) containing different amounts of RM-β-CD (molar ratio = n:1) during heating.

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Figure 9. Transmittance-temperature curves of PDMAEMA solutions (1.0 g/L, pH = 9.18) containing different amounts of RMβ-CD (molar ratio = n:1) during heating (solid line) and cooling (dashed line).

of a polymer to the copolymerization with another hydrophilic monomer.1,9 Besides affecting hydrophilicity, the inclusion complexation of RM-β-CD with polymer side group would hinder intra- and interchain hydrophobic interactions of PDMAEMA, leading to an increase of cloud point. As shown in Figure 7, the cloud point rises nonlinearly with the increase in molar ratio. In the following paragraphs, we will discuss how the inclusion complexation affects the cloud point. According to the stoichiometry of the inclusion complex, the equilibrium among the repeat unit of PDMAEMA, RM-β-CD, and inclusion complex is shown in Figure 8. We assume Cc , nC0, so that nC0 - Cc ≈ nC0. Then, we obtain eq 4 on the basis of the complexation equilibrium. Cc ¼ Keq ðC0 -Cc ÞnC0 Figure 7. Rise in cloud point with increasing the molar ratio of RM-β-CD to the repeat unit of PDMAEMA, and curve fitting with eq 5 (solid line).

Figure 8. Schematic of the complexation equilibrium of the repeat unit of PDMAEMA and RM-β-CD. Keq is the equilibrium constant. n is the molar ratio of RM-β-CD to the repeat unit of PDMAEMA.

Relationship between Inclusion Complexation and Cloud Point. We systematically investigated the thermosensitivity of a series of PDMAEMA solutions, in which the molar ratio of RM-β-CD to the repeat unit of PDMAEMA increased from 1:1 to 30:1 (Figure 6). Then, we obtained cloud points from the curves in Figure 6 and plotted the cloud point as a function of the molar ratio, as shown in Figure 7. The increase of cloud point could be because the complexation between RM-β-CD and PDMAEMA affected the hydrophilic-hydrophobic balance of a polymer chain, and made the polymer more hydrophilic. The complexation of RM-β-CD shows a similar influence on the hydrophilicity Langmuir 2010, 26(11), 8966–8970

ð4Þ

Introducing eq 3 into eq 4, we obtain eq 5. Tc ¼

ðTc0 -T0 ÞKeq nC0 þ T0 ð1 þ Keq nC0 Þ

ð5Þ

The solid line in Figure 7 is the fit curve using eq 5. The obtained fit parameters are Keq = 2.7 ( 0.3 L/mol, Tc0 = 112 ( 6 °C, and T0 = 34.4 ( 0.2 °C. The coefficient of determination (R2) is 0.998. The equation promotes understanding of the relationship between the cloud point and concentration of RM-β-CD from a thermodynamic viewpoint. It provides a theoretic guide for future research. In addition, we have confirmed the values of three parameters with Hynes’ computer program.31 Moreover, we recorded the transmittance-temperature curves of PDMAEMA solutions containing different amounts of RM-βCD during heating and cooling (Figure 9). For the polymer solution in the absence of RM-β-CD, the reversibility of phase transition is good. The addition of RM-β-CD into PDMAEMA solution results in a slight hysteresis of 1.5-2 °C between a heating run and the corresponding cooling run.

Conclusion Among R-CD, β-CD, γ-CD, and RM-β-CD, the addition of R-CD or γ-CD does not affect the cloud point of PDMAEMA solution. The cloud point of PDMAEMA solution rose gradually (31) Hynes, M. J. J. Chem. Soc., Dalton Trans. 1993, 311–312.

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with the increase of concentration of RM-β-CD. On the basis of inclusion complexation equilibrium, we deduced an equation, which clearly described the relationship between cloud point and concentration of RM-β-CD. This analysis will provide an important theoretic guide for other related research. In comparison with copolymerization with another monomer, such a supramolecular approach avoids a series of synthesis and facilitates fine-tuning of cloud point. Acknowledgment. We thank Mr. Lei Chang and Yutian Wang (Jilin University, China) for their help. This work was supported

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by Program for New Century Excellent Talents in University and National Basic Research Program (2009CB939701, 2007CB808000). Supporting Information Available: Effect of pH on cloud point, measurement of pKb of PDMAEMA, NMR titration experiment, 2D NOESY NMR experiment, and to obtain a cloud point from a transmittance-temperature curve. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(11), 8966–8970